Light chain amyloidosis (AL) is a devastating disease caused by the clonal expansion of a plasma cell that secretes a destabilized, amyloidogenic immunoglobulin light chain (LC). In the serum, these LCs aggregate into toxic oligomers and amyloid fibrils that deposit in peripheral target tissues such as the kidneys and heart causing organ malfunction and eventual death. Current AL treatments use chemotherapy and autologous stem cell transplantation to ablate the underlying diseased plasma cell population, reducing AL amyloid pathology by removing the cells secreting the destabilized, aggregation-prone LC. While this strategy is effective for 70% of AL patients, the remaining 30% of patients are too sick from LC toxicity on the heart or kidney to tolerate chemotherapy. This necessitates the development of new strategies to decrease AL amyloid pathology and allow chemotherapeutic access to the diseased plasma cells in this patient population. In the previous funding period of this award, we showed that activation of the unfolded protein response (UPR)-associated transcription factor ATF6 has significant potential to reduce AL-relevant LC toxicity through two distinct mechanisms. We found that stress-independent ATF6 activation selectively reduces the secretion and toxic aggregation of destabilized, amyloidogenic LCs. Furthermore, ATF6 activation in AL-relevant target tissues including the kidney and heart induces expression of anti-oxidant genes that protect these tissues from diverse types of oxidative stress ? the same mechanism by which LCs induce toxicity in these tissues. These results suggested the possibility that activating ATF6 using small molecules could mitigate AL amyloid pathology through two complementary, tissue-specific mechanisms: 1) reducing the secretion and subsequent toxic aggregation of amyloidogenic LCs in AL plasma cells and 2) increasing resistance of peripheral target tissues to LC-associated oxidative stress. To address this potential, we are using first-in-class compounds established during the previous funding period that selectively and robustly activate ATF6. Here, we test the hypothesis that our ATF6 activating compounds can mitigate AL amyloid pathology through two distinct, tissue-specific mechanisms. We are using these compounds to define their potential to mitigate LC-associated toxicity through complementary, multi-tissue mechanisms in both plasma cells and peripheral target tissues such as the kidney and heart. We will show that our ATF6 activators induce ER proteostasis remodeling in AL patient plasma cells to selectively reduce the secretion and toxic aggregation of amyloidogenic LCs. Furthermore, we will show that pharmacologic ATF6 activation in primary heart or kidney cells attenuates LC-associated toxicity through increased expression of anti-oxidant genes. Through these efforts, we will show that our ATF6 activators offer significant translational potential to reduce AL amyloid pathology through distinct, multi-tissue mechanisms and allow chemotherapeutic access to the underlying AL-associated plasma cells in the currently untreatable population of AL patients suffering from severe kidney or cardiac involvement.